1. Field of the Invention
The present invention relates to a circuit for controlling a power converting apparatus such as a V/f controlled PWM inverter for driving an induction motor.
2. Description of the Prior Art
It is known in the art that induction motors driven by V/f controlled PWM inverters may suffer from abnormal oscillation caused by oscillation of motor currents (see, "Application Guide to Inverter Drive", Technical Reference No. 148 of the Japan Electrical Manufacturers' Association) .
One method for limiting the oscillation of such a motor current is disclosed in an article "Limiting Current Oscillation in an Inverter-Induction Motor System", Published in the National Meeting of Institute of Electric Engineers of Japan, No. 1665, 1989. This method detects the primary currents of an induction motor, computes the amplitude of the current, picks up an oscillation component from the amplitude of the current, and corrects an output voltage command value by using the oscillation component.
As a control method of V/f controlled PWM inverters, a flux control method is known in the art (see, "High-Performance, General Use Flux Controlled PWM Inverter" IEA-87-6, Society for the Study of Industrial Electric Power Application of the Institute of Electric Engineers of Japan). This method detects terminal voltages of an induction motor via an integrator, assumes that the detected signals correspond to the primary flux of the induction motor and controls it to agree with flux command values by an adjuster, so that the primary flux is controlled to maintain a constant value.
The above-mentioned methods, however, cannot fully limit the oscillation of the motor current when the condition of load, the type and capacity of a motor and the like change.